A Monte Carlo approach for small electron beam dosimetry

Background and purpose: In treatments where it is necessary to conform the field shape yielding a very small effective beam area, dosimetry and conven tional treatment planning may be inaccurate. The Monte Carlo (MC) method ca n be an alternative to verify dose calculations. A conjunctival mucosa-asso ciated lymphoid tissues lymphoma is presented, to show the importance of an independent assessment in critical situations. Materials and methods: In this work, the MC technique has been employed usi ng the program BEAM (based on EGS4 code). Electron beam simulation has been performed and the results have been compared with those obtained with film s. The patient dose distribution has been obtained by two methods: the full Monte Carlo (FMC) simulation and a conventional planning system (PLATO). Results: Concerning dosimetry, some differences have been observed in the c omparison of profiles obtained with film and those obtained with the MC met hod. Moreover, significant differences were found in the patient isodose di stribution between both calculation methods. Conclusions: The results highlight that, in treatments where small beams ar e needed, conventional dosimetry and planning systems have some limitations . Therefore, an independent and more accurate assessment, such as MC, would be desirable. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.